Ribozymes having 2′-O substituted nucleotides in the flanking sequences

ABSTRACT

The present invention provides methods for increasing ribozyme catalytic activity without reducing specificity, which methods comprise contacting an RNA molecule with a ribozyme having a flanking sequence modified to contain 2&#39;-O-substituted nucleotides. The invention also provides ribozymes comprising a flanking sequence modified to contain 2&#39;-O-substituted nucleotides. In addition, the invention provides methods for increasing ribozyme catalytic activity comprising contacting an RNA molecule with a ribozyme having a flanking sequence modified to contain a 2&#39;-O-substituted nucleotide and a facilitator oligonucleotide. The present invention further provides compositions comprising a ribozyme having modified flanking sequences and an effective amount of a facilitator oligonucleotide.

This is a continuation of application Ser. No. 08/296,274, filed Aug.25, 1994, now abandoned, which is a continuation of application Ser. No.07/842,049, filed Feb. 26, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ribozymes that cleave RNA. More specificallyit reflects the enhancement of ribozyme catalytic activity by modifyingribozyme flanking sequence nucleotides to have substituents on theoxygen at the 2′-position, and by using a facilitator oligonucleotidecomplementary to an RNA sequence contiguous to the ribozyme.

2. Description of the Related Art

Drugs might be based on RNA catalysts or enzymes (ribozymes) designed tocleave viral or messenger RNA with high specificity at a rapid rate.These requirements historically have been mutually limiting.

Ribozymes consist of a catalytic core having flanking sequences adjacentthe core which hybridize to the substrate RNA. The simplest ribozyme isan RNA motif known as a hammerhead.

Among the factors which limit ribozyme activity in cells are the extentof cellular uptake of the ribozyme and the extent of ribozymedegradation by nucleases. Accordingly, ribozymes having increasedresistance to nuclease degradation are desired.

Ribozyme specificity depends on the number of base pairs formed betweenthe ribozyme flanking sequences and its RNA substrate. Increased basepairing has been shown to decrease the rate of cleavage. Goodchild andKohli, Arch. Biochem. Biophys., 284: 386-391 (1991). Goodchild and Kohlistudied the cleavage of a sequence from HIV-1 RNA by various hammerheadribozymes and determined that the rate of cleavage was dependent on thelength of the flanking sequence. Shorter sequences were shown to resultin weaker binding between the ribozyme and the cleavage productstogether with increased rate of cleavage. A ribozyme with 12 bases inthe flanking sequences cleaved 10 times faster then one with 20 bases.

However, to have the requisite selectivity or specifity, i.e., theability to discriminate between all RNA molecules in a cell, a ribozymemust form a minimum of about 15 base pairs with the target substrate.This requirement for selectivity limits the rate of cleavage that may berealized.

Accordingly, ribozymes having increased catalytic activity or methods ofincreasing ribozyme catalytic activity are needed.

Uhlenbeck, Nature, 328: 596-600 (1987) describes the synthesis of twooligoribonucleotides that can combine to form a structure consistentwith the consensus self-cleaving domain. Because rapid cleavage of oneof the oligomers was observed only when the other was present, thedomain was necessary and sufficient for cleavage. The properties of thecleavage reaction were studied in detail. Nearly complete cleavageoccurred even with large excess of the oligomer that was cleaved. Thisindicates that the oligomer that is uncleaved can cycle in the reactionand therefore be considered to act as a catalyst in the cleavage of theother oligomer.

Haseloff and Gerlach, Nature 334: 585-59 (1988), discuss the dissectionof the RNA substrate and enzyme activities from a single self-cleavingdomain from the (+) strand of the satellite RNA of tobacco ringspotvirus (sTobRV). Inspection of the separated substrate and ribozymeactivities, in comparison with other naturally-occurring self-cleavingdomains, led to a model for the design of oligoribonucleotides whichposses new and highly sequence-specific endoribonuclease activities.This model was successfully tested by the design and construction ofribozymes targeted against three sites within the Tn9 chloramphenicolacetyl-transferase (CAT) messenger RNA sequence.

Chemical modifications to nucleotides in the central region of varioushammerhead ribozymes have been attempted; no such modifications haveresulted in increased catalytic activity. In fact, almost all suchreported modifications have resulted in decreased catalytic activity forthe ribozymes.

Perreault, et al., Nature 344: 565-567 (1990) reports the results ofreplacing ribonucleotides in a ribozyme with deoxyribonucleotides.Analysis of the cleavage products of several of the hammerhead analogsindicated the involvement and the reaction of the 2′-OH adjacent to thecleavage site in a substrate. This analysis demonstrated that some 2′-OHgroups in the catalytic region affect activity. The introduction of2′-deoxynucleotides at the conserved positions E 13, 14 and 27-29 withinthe ribozyme sequence resulted in a 96% decrease of catalyticefficiency.

In addition, Perreault et al., Biochemistry 30: 4020-4025 (1991), andDahn and Uhlenbeck, Biochemistry 72: 819-23 (1990) report that thereplacement of various 2′-hydroxyl groups with hydrogen atoms reducedthe catalytic activity of hammerhead ribozymes.

Olsen et al., Biochemistry 30: 9735-9741 (1991), report that replacing2′-hydroxyl groups on all adenosine residues by either fluorine orhydrogen produced a large decrease in catalytic activity.

Pieken et al., Science 253: 314-317 (1991), report that catalyticactivity was reduced by replacing various 2′-hydroxyl groups onadenosine residues by fluorine and by replacing the 2′-hydroxyl groupson cytidine residues by amine groups. However, catalytic activity wasunaltered by replacing the 2′-hydroxyl groups on cytidine residues byfluorine or the 2′-hydroxyl groups on uridine residues by fluorine oramino groups.

Odai et al., FEBS Letters 267: 150-152 (1990), report that replacing byhydrogen the exocyclic amino group of a conserved guanosine residue inthe core region reduced catalytic activity.

Ruffner and Uhlenbeck, Nucleic Acids Research 18: 6025-6029 (1990), andBuzayan et al., Nucleic Acids Research 18: 4447-4451 (1990), disclosethat replacing oxygen atoms by sulfur on various internucleotidephosphate residues reduced catalytic activity.

Fedor and Uhlenbeck, Proc. Natl. Acad. Sci. USA 87: 1668-1672 (1990),analyzed the kinetics of cleavage for several hammerhead sequences tocharacterize the reaction mechanism and explore how nucleotides involvedin substrate binding affect cleavage.

Goodchild et al., Arch. Biochem. Biophys. 263: 401-409 (1988) discussesthe effects of a series of synthetic oligonucleotides (hybridons)complementary to the 5′ non-coding regions of rabbit β-globin mRNA onendogenous protein synthesis in a rabbit reticulocyte cell-freetranslation system. With highly purified hybridons inhibition wascompletely specific for beta globin. Mixtures of two oligonucleotidesbinding contiguously to the mRNA were more effective than eitheroligomer alone.

Maher and Dolnick, Nucleic Acids Res. 16: 3341-3358 (1988) report thatantisense oligonucleotides containing either anionic diester or neutralmethylphosphonate internucleoside linkages were prepared by automatedsynthesis, and subsequently compared for their ability to arresttranslation of human dihydrofolate reductase (DHFR) mRNA in a nucleasetreated rabbit reticulocyte lysate. In the case ofoligodeoxyribonucleotides, tandem targeting of three 14-mers resulted insynergistic and complete selective inhibition of DHFR synthesis at atotal oligomer concentration of 25 μM.

Kutyavin et al, FEBS Lett. 238: 35-38 (1988) report that mono- anddiphenazinium derivatives of oligonucleotides complementary to the DNAsequence adjacent to the target sequence of the addressed alkylation ofDNA significantly enhance the extent and specificity of alkylation byp-(N-2-chloroethyl-N-methylamino(benzylamido) derivatives of theaddressing oligonucleotides.

Inoue et al., Nucleic Acids Res. 15: 6131-6148 (1977) determined that(1) a 2′-O-methyl oligodeoxyribonucleotide-RNA duplex was much morestable than the corresponding oligodeoxy ribonucleotide and (2) the2′-O-methyl oligonucleotide containing duplex was not a substrate forribonuclease H.

Dunlap et al., Biochemistry, 10: 2581-2587 (1971) evaluated degradationstudies using 2′-O-methylated oligonucleotides with a crude cell-freeprotein synthesizing system known to contain a variety of nucleases anddetermined the methylated nucleotides conferred nuclease resistance tothe polymers. The 2′-O-methylated oligonucleotides were very resistantto mixtures of alkaline phosphatase, snake venom phosphodiesterase, andmicrococcal nuclease.

SUMMARY OF THE INVENTION

The present invention provides methods for increasing hammerheadribozyme catalytic activity which methods comprise introducing into aflanking sequence of the hammerhead ribozyme nucleotide modified to havea substituent on the oxygen at the 2′ position, i.e., a 2′-O-substitutednucleotide.

The invention also provides hammerhead ribozymes comprising a flankingsequence nucleotide modified to have a substituent on the oxygen at the2′ position.

In addition, the invention provides methods for increasing ribozymecatalytic activity comprising contacting an RNA molecule with a ribozymehaving a flanking sequence nucleotide modified to have a substituent onthe oxygen at the 2′ position, and a facilitator oligonucleotide.

The present invention further provides compositions comprising aribozyme having modified flanking sequences and an effective amount of afacilitator oligonucleotide.

The present invention also provides ribozymes comprising a flankingsequence modified to contain 2′-O-alkyl or acyl nucleotides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the nucleotide sequences of substrate RNA (S, SEQ ID NO:1),ribozyme (R, SEQ ID NO:2) and facilitator oligodeoxy-nucleotides F₁ [SEQID NO:3], F₂ [SEQ ID NO:4], F₃ [SEQ ID NO:5], and facilitatoroligoribonucleotide F₄ [SEQ ID NO:6] with the same sequence as F₁ [SEQID NO:3]. The site of cleavage of substrate is indicated by the arrow.Boxes indicate the positions of 2′-O-methyl nucleotides (when present).

FIG. 2A is an autoradiograph showing the results of cleavage ofradiolabelled substrate S [SEQ ID NO:1] by ribozyme R [SEQ ID NO:2]without facilitator oligonucleotide to give products P₁ and P₂containing 20 and 13 nucleotides respectively.

FIG. 2B is an autoradiograph showing the results of cleavage ofradiolabelled substrate S [SEQ ID NO:1] by ribozyme R [SEQ ID NO:2] inthe presence of facilitator oligonucleotide F₁ [SEQ ID NO:3] to giveproducts P₁ and P₂ containing 20 and 13 nucleotides respectively.

FIG. 3 is a graph of the time courses of cleavage reactions usingfacilitator oligonucleotides F₁ [SEQ ID NO:3], F₂ [SEQ ID NO:4], and F₃[SEQ ID NO:5] and a control reaction with no facilitatoroligonucleotide.

FIG. 4 is a graph showing the rates of cleavage by 2′-O-methylated andunmethylated ribozymes. Filled symbols indicate reactions with F, andopen symbols those without. Circles indicate unmethylated ribozyme,triangles represent 2′-O-methylated ribozymes.

FIG. 5 is an autoradiograph showing the results of degradation of2′-O-methylated and unmethylated ribozymes in 1% bovine fetal serum.Lane 1 is 2′-O-methylated ribozyme prior to incubation with bovine fetalserum. Lanes 2-5 show methylated ribozyme incubated with serum for 1, 6,15, and 30 minutes, respectively. Lane 6 is unmodified ribozyme prior toincubation. Lanes 7-10 are unmodified ribozymes incubated with serum for1, 6, 15, and 30 minutes, respectively.

FIG. 6 is a graph of the survival of methylated and unmethylatedribozyme in Dulbecco's Modified Eagle's Medium containing 1% bovinefetal serum at 37° C.

DETAILED DESCRIPTION OF THE INVENTION

The development of antiviral drugs based on RNA catalysts has beeninhibited by the mutually limiting requirements of high specificity andRNA cleavage rate. Increased base pairing between a ribozyme and asubstrate RNA has been shown to decrease the rate of RNA cleavage. Inorder for a ribozyme to discriminate between all RNAs in a cell, aribozyme must form about 15 base pairs with the target. However, longerflanking sequences in ribozymes is related to decreased catalyticcleavage.

Moreover, this development has been hindered by the degradation ofribozymes by nucleases which takes place in cells.

It has been discovered that hammerhead ribozyme degradation by nucleaseshas been dramatically reduced while unexpectedly increasing ribozymeactivity by modifying the ribozyme flanking sequences to contain anucleotide having substituents on the oxygen at the 2′-position of thenucleotide; i.e., by introducing a 2′-O-substituted nucleotide into theribozyme. Such a modification unexpectedly results in a significantoverall increase in ribozyme catalytic activity.

The substituents on the nucleotide 2′-oxygen may be a lower alkyl group,lower alkenyl group, a phenyl alkyl group where the alkyl is loweralkyl, a phenyl alkenyl group where the alkenyl is lower alkenyl, anacyl group, or a phenylacyl group.

By lower alkyl in the present invention is meant straight or branchedchain alkyl groups having 1-6 carbon atoms.

By lower alkenyl is meant straight or branched chain lower alkenylhaving from 1-6 carbon atoms.

By acyl is meant radicals of the formula

where R is straight or branched chain lower alkyl having 1-6 carbonatoms.

By phenyl acyl is meant radicals of the formula

where A represents straight or branched chain lower alkylene having 1-6carbon atoms; and

R₁ represents phenyl.

The modified ribozymes of the present invention may be prepared onautomated oligonucleotide synthesizers or enzymatically from templates.For example, the partially 2′-O-alkylated or acylated ribozymes may beprepared using an automated solid-support phosphoramidite method withcommercial nucleoside phosphoramidites having either 2′-O-silyl,2′-O-alkyl, or 2′-O-acyl groups. The modified ribozymes of the inventionshould be prepared such that the 2′-O-substituted nucleotides arelocated primarily in the flanking sequences. Those skilled in the artwould recognize methods for preparing other 2′-O-substitutednucleotides.

Suitable synthetic methods for preparing various 2′-O-substitutednucleotides are disclosed by Iribarren et al., Proc. Natl. Acad. Sci.87: 7747-7751 (1990); Sproat et al., Nucleic Acids Res. 19: 733-738(1991); and Sproat et al., Nucleic Acids Res. 18: 41-49 (1989). Theseroutes typically involve protecting the 3′ and 5′-hydroxyl groups, andsubsequently treating with an alkyl iodide in the presence of a strongorganic base such as2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorphorin(BDDDP). Other alkylating agents such as diazomethane can also be used.In some situations, it will be necessary to protect reactive aminogroups prior to alkylation or acylation.

By flanking sequence in the present invention is meant the sequence ofnucleotides immediately adjacent to the core region of the ribozyme. Theflanking sequence is that region of the ribozyme that base-pairs withthe RNA substrate. Each ribozyme flanking sequence may comprise fromabout 3 to about 50 nucleotides.

The ribozymes modified to comprise 2′-O-substituted nucleotides may haveeither one or both of the flanking sequences modified to contain thesenucleotides. Preferred modified ribozymes will have a 2′-O-substitutednucleotide in both franking sequences. Furthermore, each flankingsequence may comprise a single 2′-O-substituted nucleotide or maycontain a plurality of these nucleotides. The number of modifiednucleotides in the flanking sequence will vary from about 20% of thenucleotides being methylated to about 100%. A more preferred range isfrom 50% to about 100%.

It has also been unexpectedly discovered that rate of cleavage ofsubstrate RNA by a modified ribozyme is further enhanced by introducingan oligonucleotide into the system which hybridizes immediately adjacentto the modified ribozyme. The facilitator oligonucleotides of thepresent invention are designed to bind to RNA sequences contiguous toflanking sequences of the ribozymes.

The facilitator oligonucleotides suitable for use in the instantinvention may be either oligodeoxynucleotides or oligoribonucleotides.Furthermore, the facilitator oligonucleotide may be selected to bind toa sequence contiguous to the flanking sequence either at the 5′ or the3′ side of the ribozyme. In addition, a combination of two facilitatoroligonucleotides may be employed, where one facilitator is boundcontiguously to the 3′ flanking sequence and the other to the 5′flanking sequence. Alternatively, a plurality of facilitators may beemployed to catalyze ribozyme activity. For example, in a systememploying three facilitators, two facilitators could bind contiguouslyto the 3′ flanking sequence, while a single additional facilitator couldbind contiguously to the 5′ flanking sequence. A variety of othercombinations are possible.

The facilitator oligonucleotides of the present invention typicallycomprise between about 5 and 50 nucleotides. More preferred facilitatoroligonucleotides comprise between about 5 and 15 nucleotides.Particularly preferred facilitators according to the invention compriseabout 13 nucleotides. Selection of a facilitator of a specific length isrelated to the length of the ribozyme flanking sequences.

In addition, facilitator deoxynucleotides may be selected to havebetween about 5 and 50 nucleotides complementary to the RNA substratesequence as well as additional nucleotides which are not complementaryto the RNA sequence.

The specific facilitator oligonucleotides are synthesized to bind to thedesired RNA sequences such that they are contiguous to the flankingsequences of various ribozymes that cleave related RNA sequences. Asdescribed here, the oligonucleotides can be synthesized on automated DNAsynthesizers or from DNA templates.

The facilitator oligonucleotides may be synthesized such that they arenot completely contiguous to the flanking sequence of the desiredribozyme. For example, the facilitator may be synthesized such that,when the ribozyme and facilitator oligonucleotide are bound to thesubstrate RNA, a small gap of from one to about five oligonucleotidesexists between the ribozyme and the facilitator oligonucleotide. Inpreferred embodiments of the invention, the gap between the facilitatorand the ribozyme will be between 0 (zero) and 2 nucleotides. The mostpreferred embodiment will have no nucleotide gap between the facilitatorand the ribozyme.

The facilitator oligonucleotides may be synthesized and subsequentlymodified to include moieties which will influence the rate of substratecleavage by ribozyme, increase uptake by cells, or increase resistanceto degradation.

By increasing the number of bases of the substrate RNA bound near thecleavage site, facilitators permit use of faster acting ribozymes withshorter flanking sequences. In viral applications, facilitators might beof dual benefit in also directing cleavage of the viral RNA byendogenous ribonuclease H.

The present invention further provides compositions comprising amodified ribozyme.

The present invention also includes compositions which comprise aribozyme and an effective amount of a facilitator oligonucleotide. Inany treatment, however, the compositions comprising the 2′-O-methylatedribozyme, or the methylated ribozyme and facilitator oligonucleotidemust be administered to individuals in a manner capable of deliveringthe oligonucleotide and ribozyme initially into the blood stream andsubsequently into cells.

The compositions of the invention may be administered parenterally,orally, by inhalation or spray or rectally in dosage unit formulationscontaining conventional non-toxic pharmaceutically acceptable carriers,adjuvants and vehicles. The term “parenteral” as used herein includessubcutaneous injections, intravenous, intramuscular, intrasternalinjection or infusion techniques. The compositions of the inventionwould be provided in a pharmaceutical formulation comprising thecomposition and a pharmaceutically acceptable carrier. In order for thecompositions to be suitable for oral administration, oligonucleotidesand ribozymes must be resistant to nucleases. Such resistance tonucleases may be imparted to the oligonucleotides by, for example,internucleotide phosphate modifications. Additional resistance tonucleases may also be bestowed on the modified ribozymes byinternucleotide phosphate modification. Modified internucleotidephosphates suitable for use in the facilitator oligonucleotides and2′-O-substituted ribozymes of the present invention includephosphorothioates, methylphosphonates, phosphoramidates, andphosphotriesters.

The amount of active composition that may be combined with the carriermaterials to produce a single dosage form will vary depending upon thehost treated and the particular mode of administration. It will beunderstood that the specific dose level for any particular patient willdepend upon a variety of factors including the activity of the specificcomposition employed, the age, body weight, general health, sex, diet,time of administration, route of administration, severity of theparticular disease undergoing therapy.

One skilled in the art will recognize that modifications may be made inthe present invention without deviating from the spirit or scope of theinvention. The invention is illustrated further by the followingexamples which are not to be construed as limiting the invention orscope of the specific procedures described herein.

EXAMPLE 1

1. Preparation of RNA Substrate

A synthetic RNA substrate strand (S, SEQ ID NO:1) was prepared tocorrespond to the sequence 146-173 in HIV-1 RNA. This RNA substratestrand was transcribed from synthetic DNA templates following a methoddescribed by Milligan and Uhlenbeck, Nucleic Acids Res. 15: 8783-8798(1987), in a reaction containing Tris·HCl (40 mM, pH 8.1), MgCl₂ (6 mM),spermidine (1 mM), dithiothreitol (50 mM), bovine serum albumin (50 μgper ml), inorganic pyrophosphatase (4 units per ml), T7 RNA polymerase(4000 units per ml) and four ribonucleotide 5′-triphosphates (1 μM each)supplemented with α-³²P-UTP (3000 Ci/mmol). After incubation at 37° C.for 2 hours, the RNA was purified by electrophoresis in 10%polyacrylamide gels containing 8 M urea. The radiolabeled RNA wasquantitated using the specific activity of the incorporated ³²P.

2. Preparation of Hammerhead Ribozyme

A hammerhead ribozyme (R, SEQ ID NO:2) designed to cleave RNA substratestrand S was prepared. The hammerhead ribozyme was prepared by automatedchemical synthesis using standard phosphoramidite reagents.2′-O-methylated ribozymes were prepared using the automated solidsupport phosphoramidite method with commercially available nucleosidephosphoramidites bearing either 2′-O-silyl or 2′-methyl groups asdescribed by Sproat et al., Nucleic Acids Res. 17: 3373-3386 (1989). Inaddition, a 2′-deoxy derivative of ribozyme R [SEQ ID NO:2] was preparedhaving all the 2′-hydroxyl groups in the flanking sequences replaced byhydrogen. Products were purified by electrophoresis in 15%polyacrylamide gels containing 8M urea, eluted by crush and soak in 0.5Mammonium acetate, desalted and quantitated by UV absorption.

3. Preparation of Facilitator and Control Oligonucleotides

Facilitator oligonucleotides F₁ [SEQ ID NO:3], F₂ [SEQ ID NO:4], and F₃[SEQ ID NO:5] were prepared to contain 13, 10, and 6 nucleotidesrespectively, and to hybridize to substrate S contiguously with ribozymeR [SEQ ID NO:2]. Facilitator oligoribonucleotide F₄ [SEQ ID NO:6] wasprepared with the same sequence as F₁ [SEQ ID NO:3]. In addition, acontrol oligonucleotide having a random sequence was synthesized. Thesequences of the facilitator oligonucleotides are shown in FIG. 1.

Both ribo- and deoxyribooligonucleotides were prepared by automatedchemical synthesis utilizing essentially the same procedures set forthin part 2 of this Example.

EXAMPLE 2

1. Cleavage of Substrate RNA with Unmethylated Ribozyme

The cleavage of substrate RNA by ribozyme R [SEQ ID NO:2] was studiedboth with and without facilitator oligo F₁ [SEQ ID NO:3]. The cleavageof substrate RNA gave products P₁ and P₂ having chain lengths expectedfrom cleavage at the site indicated in FIG. 1.

The cleavage reactions were run as follows: a solution (45 μl)containing substrate (13.4 μM), ribozyme (0.67 μM) and facilitator whereappropriate (20 μM) in 50 mM Tris·HCl (pH 7.4) was brought to 37° C.Reaction was initiated by the addition of MgCl₂ (5 μL, 200 mM). Aftertimes of 0.5, 1, 2, 5, and 10 minutes, aliquots of 5 μL were added to 15μl of saturated urea:200 mM EDTA (1:1) and cooled to about −70° C. withdry ice to stop the reaction. The samples were then denatured by heatingin formamide loading buffer at 90° C. for 3 minutes and subsequentlyanalyzed alongside molecular weight markers by electrophoresis in 15%polyacrylamide gel containing 7M urea. The products wereautoradiographed. The autoradiographs are shown in FIG. 2. Panel A showsthe results of the cleavage reaction without any facilitatoroligonucleotide and Panel B shows the results of cleavage withfacilitator oligo F₁ [SEQ ID NO:3] present.

2. Cleavage of Substrate RNA with 2′-O-methylated Ribozyme

The cleavage of substrate RNA was studied using methylated andunmethylated ribozymes with and without facilitator F₁ [SEQ ID NO:3].

The cleavage reactions were run under substantially the same conditionsas described above in part 1 of this example. Reactions contained 20equivalents of substrate and 30 equivalents of facilitator relative tothe methylated or unmethylated ribozyme.

As shown in FIG. 4, the half-lives of the substrate without F₁ [SEQ IDNO:3], were 10 minutes with unmethylated ribozyme and 2 minutes withmethylated ribozyme. When facilitator F₁ [SEQ ID NO:3] was utilized,half-lives were further reduced to 1.3 minutes and 0.5 minutes,respectively.

3. Cleavage of substrate RNA with 2′-Deoxy Ribozyme

The cleavage of substrate RNA was studied using the 2′-deoxy derivativeof ribozyme R [SEQ ID NO:2] (prepared above in Example 1, part 2).Cleavage reactions were run under substantially the same conditions asdescribed above in part 1 of this example. Under these conditions, theunmethylated parent ribozyme cleaved about 80% of the substrate;2′-deoxy derivative cleaved only about 8% of the substrate.

EXAMPLE 3

Relation of Facilitator Length to Ribozyme Activity

Cleavage of substrate RNA by ribozyme R was determined in the presenceof facilitator oligonucleotides (F₁ [SEQ ID NO:3], F₂ [SEQ ID NO:4], F₃[SEQ ID NO:5], and F₄ [SEQ ID NO:6]) of varying length. Cleavagereactions were run under conditions substantially similar to thoseemployed in Example 2 above. Products and starting materials werequantitated for each time point. Autoradiograph gels were sliced and thematerials on the slices quantitated by scintillation counting. Theresults of this experiment are graphically shown in FIG. 3.

Cleavage with no facilitator reached about 94% completion after about160 minutes. The facilitator of 13 deoxynucleotides significantlyreduced substrate half life. Table 1 shows the time required forribozyme to cleave 10 equivalents of substrate at 37° C. The longestfacilitator, F₁ [SEQ ID NO:3], reduced the half-life of substrate from10 minutes to 1.3 minutes. The effects of facilitators F₁-F₃ [SEQ IDNOS:3-5] were inversely related to their lengths. A controloligonucleotide of the same length as F₁ [SEQ ID NO:3] had no effect onthe rate.

In a separate experiment, it was found that oligodeoxyribonucleotide F₁was more effective at catalyzing ribozyme activity thanoligoribonucleotide F₄ [SEQ ID NO:6] having the same sequence.

TABLE 1 Half-Lives of Substrate in the Presence of Ribozyme andFacilitators Facilitator [S]_(o) ¹ Half-Life (min) none 2.7 10 F1[SEQ IDNO:3] 2.7 1.3 F2[SEQ ID NO:4] 2.7 1.9 F3[SEQ ID NO:5] 2.7 6.9 none 0.940 F1[SEQ ID NO:3] 0.9 4.9 F4[SEQ ID NO:6] 0.9 12.3 ¹Startingconcentrations of substrate (μM)

From the foregoing, it will appreciated that although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit or scope of the invention.

EXAMPLE 4

Ribozyme Resistance to Nuclease Degradation

The effect of ribozyme nucleotide modification on resistance to nucleasedegradation was examined. Unmodified and 2′-O-methylated hammmerheadribozymes (prepared in Example 1, part 2) were incubated in Dulbecco'sModified Eagle's Medium containing 1% bovine fetal serum at 37° C.Aliquots were withdrawn, analyzed by polyacrylamide gel electrophoresis,and autoradiographed. Bands were quantitated by scanning densitometry.The results are depicted in the autoradiograph shown in FIG. 5.Methylation of the 2′-oxygen in flanking sequence nucleotides reducedthe rate of degradation of the ribozyme about four fold. Half-lives ofthe methylated (3 minute) and unmodified (0.8 minutes) ribozymes weredetermined from plots of the results, as shown in FIG. 6.

8 1 33 RNA Human immunodeficiency virus type 1 1 gaaaacagac ccuuuuagucaguguggaaa auc 33 2 35 RNA Artificial Sequence Description of ArtificialSequenceHammehead ribozyme 2 gcacacucug augaggccgu uaggccgaaa cuaaa 35 313 DNA Artificial Sequence Description of Artificial SequenceFacilitatoroligonucleotide 3 agggtctgtt ttc 13 4 10 DNA Artificial SequenceDescription of Artificial SequenceFacilitator oligonucleotide 4agggtctgtt 10 5 6 DNA Artificial Sequence Description of ArtificialSequenceFacilitator oligonucleotide 5 agggtc 6 6 13 RNA ArtificialSequence Description of Artificial SequenceFacilitator oligonucleotide 6agggucuguu uuc 13 7 32 RNA Artificial Sequence Description of ArtificialSequenceHammerhead ribozyme 7 gcacacucug augccguuag gccgaaacua aa 32 835 RNA Artificial Sequence Description of Artificial SequenceHammerheadribozyme 8 gcacacucug augaggccgu uaggccgaaa cuaaa 35

What is claimed is:
 1. A hammerhead ribozyme having the sequences 5′ GCACAC UCU GAU GCC GUU AGG CCG AAA CUA AA 3′ SEQ ID NO 7 where thenucleotides forming the 5′ CACU 3′ and 5′ CUAA 3′ sequences of theribozyme are 2′-O-methyl nucleotides.
 2. A method for increasing thecatalytic activity of a ribozyme having the sequence 5′ GCA CAC UCU GAUGAG GCC GUU AGG CCG AAA CUA AA 3′ SEQ ID NO 8 comprising preparing theribozyme to have 2′-O-methyl nucleotides in the 5′ CACU 3′ and 5′ CUAA3′ sequences of the ribozyme.
 3. A hammerhead ribozyme comprising one ormore 2′-O-methyl substituted nucleotides in one or both of theribozyme's flanking regions.
 4. The hammerhead ribozyme according toclaim 3, wherein the ribozyme's flanking region is from 3 to 10nucleotides long.
 5. The hammerhead ribozyme according to claim 3,wherein 20% or more of the nucleotides in the flanking sequences are2′-O-methyl substituted.
 6. The hammerhead ribozyme according to claim3, wherein from 50% to 100% of the nucleotides in both flanking regionsare 2′-O-methyl substituted.
 7. A composition comprising a ribozymeaccording to claim 3 and a facilitator oligonucleotide.
 8. Thecomposition according to claim 7, wherein the facilitatoroligonucleotide hybridizes with a sequence in a target RNA that isspaced no more than about three nucleotides from the target RNA to whichthe ribozyme hybridizes.
 9. The composition according to claim 8,wherein the facilitator oligonucleotide is from about 5 to about 15nucleotides long.
 10. A hairpin ribozyme comprising one or more2′-O-methyl substituted nucleotides in the ribozyme's flanking region.11. The hairpin ribozyme according to claim 10, wherein the ribozyme'sflanking region are from 3 to 10 nucleotides long.
 12. The hairpinribozyme according to claim 10, wherein 20% or more of the nucleotidesin the flanking region are 2′-O substituted.
 13. The hairpin ribozymeaccording to claim 10, wherein from 50% to 100% of the nucleotides inthe flanking region are substituted.
 14. A composition comprising aribozyme according to claim 10 and a facilitator oligonucleotide. 15.The composition according to claim 14, wherein the facilitatoroligonucleotide hybridizes with a sequence in a target RNA that isspaced no more than about three nucleotides from the target RNA to whichthe ribozyme hybridizes.
 16. The composition according to claim 15,wherein the facilitator oligonucleotide is from about 5 to about 15nucleotides long.